1. Field of the Invention
The present invention generally relates to a reverberation system for imparting reverberant sound to real sound from a real sound source location, which reverberant sound is produced within a virtual acoustic room that is surrounded by virtual walls and ceiling (that will be collectively called "virtual room partitions") that are assumed to be present at locations where no actual walls or ceiling exist, as if the sound from the real sound source was reflected by the virtual room partitions. In particular, this invention is concerned with such a reverberation system that is able to change the shape of the virtual acoustic room as desired, by changing the location of the virtual room partitions.
2. Prior Art
In outdoor concert halls, seats for audience are not surrounded by walls and ceiling that would be present in the case of indoor concert halls, and, therefore, no reverberation effect arises from reflected sound as produced in indoor halls unless the outdoor halls are designed otherwise. A known reverberation system images a virtual hall surrounded by virtual walls that are assumed to be present at locations of such an outdoor concert hall where no walls, ceiling, or the like, actually exist, and creates reflected sound in the virtual hall as if sound from a real sound source location were reflected by the virtual walls. The reverberation system may also be utilized when imaging a virtual hall of a small volume or capacity within an indoor hall of a large volume or capacity. This type of reverberation system generally includes a main loudspeaker that generates sound from a real sound source location, a plurality of sub-loudspeakers for producing reverberation or reverberant sound, which are arranged, at some intervals, around the virtual walls that define the virtual hall including the real sound source location, and a virtual reverberant sound synthesizer for synthesizing or producing virtual reverberant sound signals based on which reflected sounds are generated from the sub-loudspeakers as if the sound from the real sound source location were reflected by the virtual walls.
A specific example of the reverberation system as disclosed in Japanese Patent No. 2569872 was developed based on a fundamental concept as follows. In an actual hall 200 as shown in FIG. 1, sound generated from a real sound source 134 travels the shortest distance to reach a sound receiving point 136 as direct sound 150 and also reaches the sound receiving point 136 after being reflected once or a plurality of times by walls 138. An imaginary sound source 140 is assumed to be located at a point at which an extension of a line connecting the sound receiving point 136 and the final reflection point of the reflected sound 160 that reaches the sound receiving point 136 intersects with an extension plane of the rear wall including the rear sound source 134, and this imaginary sound source 140 is recognized as if it generated the reflected sound 160 as a direct sound. In the example of FIG. 1, reference numerals 140-1, 140-2, . . . denote a plurality of imaginary sound sources, and the reverberant sound structure (impulse response) as observed at the sound receiving point 136 is determined depending upon the positions of these imaginary sound sources.
In a virtual hall 300 as shown in FIG. 2, on the other hand, no physical wall really exists, but instead a virtual wall 138' is assumed to cause reflection of sound. To construct a virtual hall 300 through simulation, sub-loudspeakers 144-1, 144-2, . . . are positioned at some intervals so as to generate individually synthesized, virtual reverberant sounds, from a plurality of simulation positions in the vicinity of the virtual wall 138', and are each oriented in a direction in which the reflected sound is reflected.
The virtual reflected sound 170 emitted from each sub-loudspeaker 144 as described above is synthesized through digital processing. More specifically, the digital processing is performed using a plurality of digital filters, more particularly, non-recursive FIR (Finite Impulse Response) filters, each of which incorporates reflected sound parameters (time delay, amplitude and others) having a reflected sound structure of an impulse response that is almost identical with an impulse response observed at each of the above simulation positions (or obtained by computing based on CAD data or the like). A source signal corresponding to the sound emitted from the real sound source is fed to each of these filters, to be processed according to a convolution algorithm using the reflected sound parameters, to thereby produce virtual reverberant sound signals for the respective simulation positions.
The above-described system as disclosed in Japanese Patent No. 2569872 solves a problem of previously available reverberation systems that the optimum sound receiving point is theoretically limited to a single point (namely, the reflected sound structure is determined assuming only one sound receiving point), by providing a system arrangement capable of securing a wide sound receiving area. The basic concept of the disclosed system resides in that the reflected sound from one imaginary sound source 140 is reproduced from a plurality of loudspeaker devices 144-1, 144-2, . . . with time differences and level differences, and the reproduction is conducted with respect to each of a plurality of imaginary sound sources 140-1, 140-2, . . . .
In the above-described reverberation system of Japanese Patent No. 2569872, reflected sound parameters (impulse responses) assuming one virtual hall are incorporated in advance in each digital filter. It is, therefore, difficult to change reverberation characteristics to be simulated, according to the shape of a desired acoustic room (virtual hall). Since virtual acoustic rooms having different shapes possess different reverberation characteristics, the impulse responses to be adopted need to be re-measured for each shape of virtual acoustic room, or a plurality of sets of impulse responses that match or fit virtual acoustic rooms having typical shapes are prepared in advance, so that an appropriate set of impulse responses can be selectively used in accordance with the shape of a desired virtual hall. In either case, the system structure or arrangement is likely to be complicated. Where a large number of loudspeakers are used, in particular, the difficulty in changing the simulated reverberation characteristics is further increased.